Some energy storage systems (e.g., battery packs) pump a cooling fluid through a cooling tube in between or near the energy-storing cells. One aspect of the thermal architecture of such an energy storage system is the thermal resistance between the cells and the heat transfer fluid contained within the cooling tube. Thermal interface materials (TIMs) can be used for bridging the physical gap between the cell and the cooling tube. In the past, cells have been populated inside a plastic housing, a straight cooling tube has been run between cell rows, and the interior has been flooded with a thermally conductive epoxy. Such epoxy may be high-cost and high-mass, with large conduction path lengths. In another approach, appropriate thermal resistance has been obtained using a high-cost, compressible silicone sponge TIM applied to a scalloped cooling tube.